[RFC] Make _FILE_OFFSET_BITS=64 default.

[Roland McGrath]

> It can be an ABI change to other people's sources as if they used
> off_t, now they have a different type.

Yes, that is part of what an API change means: recompiling might produce a
different ABI.  That is one of the reasons why API changes require careful
consideration.  When we talk about an ABI change on this list, we are
talking about the ABIs of libraries our package installs.